Whisk milk, water, cornmeal, 1 teaspoon salt, and ½ teaspoon pepper in large microwave – safe bowl. Cover with plastic wrap and microwave on high power until cornmeal is thick and creamy, 15 to 22 minutes, whisking cornmeal and replacing wrap halfway through cooking.

Meanwhile, melt butter in large saucepan over medium heat. Add leeks and corn and cook until softened, about 6 minutes. Transfer half leek corn mixture to bowl with basil. Cover to keep warm.

Stir microwaved polenta into saucepan with remaining leeks and corn. Whisk in mascarpone and parmesan cheese. Simmer over medium heat until creamy and mascarpone is heated through about 1 minutes. Season with salt and pepper. Divide polenta among individual bowls and top with reserved leek corn mixture. Serve.